Semiconductor devices such as integrated circuits, solid state sensors and flat panel displays are produced by microlithographic processes in which a photoresist is used to form a desired feature pattern on a device substrate. Light is passed through a patterned mask onto the photoresist layer which has been coated onto the device substrate. A chemical change occurs in the light-struck areas of the photoresist, causing the affected regions to become either more soluble or less soluble in a chemical developer. Treating the exposed photoresist with the developer etches a positive or negative image, respectively, into the photoresist layer. The resulting pattern serves as a contact mask for selectively modifying those regions of the substrate which are not protected by the pattern. These modifications may include, for example, etching, ion implantation, and deposition of a dissimilar material.
Plasma etching processes are being used increasingly to transfer photoresist patterns into device substrates or underlying layers. It is important that the photoresist pattern does not erode excessively during the plasma etching process, otherwise, the precise pattern will not be transferred into the underlying layer. For example, if the photoresist is removed by the etching process before the underlying layer has been fully etched, then the feature size will begin to increase as the etching process proceeds since the photoresist is no longer protecting the area of the substrate which it once covered. Because of the precision required in etching this is disadvantageous.
Plasma etching processes for organic layers such as color filters used in solid state color sensors and flat panel displays require photoresist materials with exceptional resistance to oxygen plasma etching. Conventional photoresists cannot be used effectively since they are primarily mixtures of organic compounds and resins which are etched more rapidly than the color layers.
Silicon-containing photoresists and silylated photoresist products have been developed to provide greater etch selectivity when patterning organic layers by oxygen plasma etchinig. Such compositions have been described, for example, in U.S. Pat. No. 5,250,395 issued to Allen et al. and by F. Coopmans and B. Rola in Proceedings of the SPIE, Vol. 631, p. 34 (1986). During the etching process, the silicon components in the photoresist are rapidly converted to silicon dioxide which resists further etching. The in-situ formed silicon dioxide layer then becomes the mask for etching the underlying organic layer.
Although oxygen is normally the principal plasma etchant for organic layers, it may be admixed with various fluorinated gases such as NF.sub.3, C.sub.2 F.sub.6, HCF.sub.3, and CF.sub.4 to aid in the removal of inorganic residues arising from metallic species. The inorganic residues are often present, for example, in color filter layers. Since, silicon dioxide is readily etched by fluorine-containing plasma etchants, silicon-containing photoresists cannot be used effectively in plasma etching processes where fluorine-containing gases are present.
For color filter applications, it is also desirable that the plasma etch-resistant photoresist be left in place as a permanent part of the device structure after the etching process has been completed. To function suitably in this respect, the remaining photoresist material must be continuous, homogeneous, highly adherent, and optically clear so that it does not reduce the transmissivity of the color filter assembly. Silicon-containing photoresists often exhibit poor optical clarity after plasma etching or high temperature thermal treatments which are usually applied to color filter layers. This problem is especially prevalent with silylated phenolic photoresists since the phenolic components form highly colored species when heated to above approximately 125.degree. C.
There are other microlithographic applications where it is also desirable to leave a processed photoresist layer in place as a permanent device structure. For example, thin barium titanate and lead zirconate titanate layers are needed as ferroelectrics in advanced memory devices. Presently, these complex metal oxides must be deposited by chemical vapor deposition and then patterned in separate plasma etching processes. Considerable time and expense could be saved if the materials could be applied in a photosensitive precursor form by spin coating and then directly patterned by a photolithographic process, after which the patterned layer would be calcined in air to form the desired metal oxide device features.
Accordingly, there is a need for a photoresist material with greater plasma etching resistance in processes utilizing oxygen and/or fluorinated gases as the etchant species. At the same time, there is a need for a photoresist which is convertible to a permanent metal oxide layer with chemical, thermal, electrical, and optical properties useful for device application and whose properties can be controlled by adjusting the composition of the photoresist. The photoresist should further possess high resolution patterning capabilities and be easily integrated into modern microlithographic processing schemes. We have now discovered that all of these requirements surprisingly can be met with a photoresist composition containing an addition polymerizable organotitanium polymer as the principal constituent.